A configuration disclosed in Japanese Patent Application Publication No. 2000-285892 A and so on, may be cited as a conventionally employed battery case lid of this type. FIG. 8 is an exploded perspective view showing a battery case using a conventional lid. In the drawing, a lid 1 constitutes a battery case 3 together with a case main body 2 having a closed-end tube shape. The lid 1 and the case main body 2 are formed from metallic sheet made of aluminum, an aluminum alloy, nickel-plated steel, stainless steel, or the like. The lid 1 is provided with a lid main body 4 and a safety valve 5. The lid main body 4 is a planar portion formed in a rectangular shape having a short side 40 and a long side 41 when seen from above. The safety valve 5 is provided to prevent the battery case 3 from bursting open when an internal pressure of the battery case 3 (referred to hereafter as internal case pressure) rises. When the internal pressure of the battery case 3 exceeds a predetermined value, the safety valve 5 ruptures, thereby releasing the internal pressure of the battery case 3 to the outside. The safety valve 5 includes an annular thin portion 50 formed integrally with the lid main body 4 by performing coining (pressing) in which an engraving punch is pressed against the metallic sheet serving as a base material of the lid 1. When the internal pressure of the battery case 3 exceeds the predetermined value, all or a majority of the annular thin portion 50 fractures, thereby rupturing the safety valve 5.
The inventors of the present application discovered, while repeatedly manufacturing prototypes of the battery case 3 described above and testing operation of the safety valve 5, that when coining is used to form the annular thin portion 50 integrally with the lid main body 4 made of metallic sheet, a unique problem occurs.
FIG. 9 is an illustrative view showing deformation of the lid 1 shown in FIG. 8 when the internal case pressure rises. As shown in FIG. 9, when the internal case pressure rises, the lid main body 4 of the lid 1 deforms in an arc shape such that a ridge 4c extending in a long direction 4b of the lid main body 4 is formed substantially centrally in a short direction 4a of the lid main body 4. As a result, tensile stress acting on the annular thin portion 50 of the safety valve 5 disposed in a central portion of the lid main body 4 is greater in the short direction than in the long direction. Meanwhile, an inner peripheral region of the annular thin portion 50 is increased in thickness by the coining process, and therefore, when the internal case pressure rises, the inner peripheral region of the annular thin portion 50 deforms more slowly than the entire lid main body 4. As a result, excessive stress is concentrated in a part (a part on the long side 41 side of the lid main body 4) of the annular thin portion 50 removed from the ridge 4c. 
When the stress acting on the annular thin portion 50 is excessively concentrated in a part of the annular thin portion 50, only this part fractures. In this case, the internal pressure of the battery case 3 escapes through the fractured part, and therefore the safety valve 5 cannot rupture normally. As a result, the speed at which the internal case pressure is released decreases (this condition is termed as a slow leak). In other words, with the conventional configuration described above, no measures are taken to avoid stress concentration accompanying deformation of the lid main body 4 when the internal case pressure rises, and as a result, the safety valve 5 may be unable to rupture normally.